Plot hole (spoilers)
Silas is working for someone he only knows as The Teacher. He doesn't know who the Teacher really is so as far as he knows it could be anyone. He breaks into Teabing's home prepared to kill everyone including Teabing having no idea that Teabing is really the man he is working for. So Teabing could have been killed by the very man he sent to find the keystone, ending his own life's quest to expose the holy grail.
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